1. Field of the Invention
The present invention relates to devices and methods for power control in electronic devices. More specifically, the present invention includes a method and apparatus for controlling power battery powered electronic devices.
2. The Relevant Art
Personal computing has undergone a revolution over the past decade. The steady increase in the computing power of microchips, coupled with advances in the miniaturization of electronics and the development of strong, lightweight materials and batteries, has fostered the creation of a large market in ever smaller computing devices. Such computing devices include notebook computers, such as Apple Computer, Incorporated's PowerBook.TM. and Duo.TM. notebook computers, which are lightweight and offer the computing power of desktop computers. Smaller still are the relatively new hand held computing devices, otherwise known as personal digital assistants, or PDAs. These devices are exemplified by the Apple Newton.TM. PDA and the Sharp Wizard PDA, both of which offer users the ability to enter information electronically by writing on an electronic tablet using a stylus. Such devices have become popular replacements for traditional personal organizers such as date books and note pads.
One difficulty with developing small computing devices arises from the limitations of battery technology. Although batteries are available which are both lightweight and powerful, their lifetimes are on the order of hours imposing undesirable limits on the usefulness of notebook computers and PDAs. Thus, designers are challenged to develop small computing devices which draw battery current as efficiently as possible to extend their utility. One approach to this problem has been to minimize power intensive Central Processor Unit (CPU) operations by the inclusion of Application-Specific Integrated Circuits (ASICs) which handle many tasks that otherwise would use CPU time in an energy inefficient manner, and the design of systems which only require the CPU to operate when absolutely necessary. The latter approach is described in co-pending U.S. Patent application Ser. No. 08/316,040, filed concurrently herewith and which is incorporated herein by reference. This is especially difficult in low power systems such as PDSs as there are many sources of power failure and power-off conditions that cannot always be handled by software.
One particularly troublesome source of battery drain is related to the General Purpose Input/Output (GPIO) circuits which reside on the ASIC and which circuits control the power allocation to various internal and external devices which interface with the ASIC. Such devices include the display screen, the tablet (in the case of a stylus-based system such as the Newton.TM. PDA), and Personal Computer Memory Card Industry Association (PCMCIA) cards which add a variety of functionalities to computers such as additional memory and modem capability. Power loss related to PCMCIA cards can be especially difficult to control in situations wherein a user attempts to install a PCMCIA card that draws too much current from the system, forcing an unanticipated powerdown. Present system designs do not provide a mechanism to advise the user that the offending card cannot be accommodated under the present system conditions and/or ensure that functionalities which should remain powered, e.g., PCMCIA cards maintaining a communications link with a remote source, remain powered. Thus, under the limitations of present GPIO design, the user is vulnerable to system failure caused by a PCMCIA card or other device which causes an unanticipated system powerdown, i.e., a powerdown in response to an event which causes an error state in the system for which no software "trap" is available, resulting from the device demanding too much current from the system. In addition, present GPIO designs tend to "leak" current to ground, a condition which can be ill afforded in PDAs having limited battery life. Thus, there is a need for more "intelligent" power management of GPIOs, especially under error conditions.